1.What are the most severe and direct effects of attenuation?
Coating/Oil Layer Failure: Cold-rolled coils are typically coated with rust-preventive oil or covered with a temporary protective film (such as a peelable plastic film) at the factory. Under prolonged exposure to sunlight:
UV rays will cause the protective film to age, become brittle, and peel off.
High temperatures will cause the rust-preventive oil to evaporate, dry out, or flow unevenly, losing its protective effect.
Accelerated Electrochemical Corrosion: Once the protective layer fails, the exposed steel surface will rapidly undergo electrochemical corrosion in a humid environment exposed to sunlight and rain, producing red rust. This will lead to:
Surface Quality Defects: For applications requiring high surface quality (such as automotive exterior panels and appliance panels), rust means the material is unusable.
Thickness Reduction and Strength Decrease: Severe rust will effectively reduce the material's thickness, lowering its load-bearing capacity and mechanical properties.
Impact on Subsequent Processing: The rust layer will affect the adhesion of subsequent processes such as spraying and electroplating.
2.What are the potential degraded effects on mechanical and processing properties?
High-Temperature Aging and Grading: Prolonged exposure to high daytime temperatures (surface temperatures of steel plates can reach 60-70°C or higher under summer sun) accelerates the aging process of materials.
For some low-carbon steels, especially those that have undergone work hardening through cold rolling, prolonged exposure to certain temperatures may cause slight changes (usually an increase) in yield strength and hardness, while reducing plasticity and toughness, leading to an increased risk of cracking during subsequent bending and stamping.
Internal Stress and Deformation Risk: This is similar to the principle of low-temperature storage discussed earlier, but the situation is more complex. Drastic temperature cycles caused by day-night alternation (such as daytime heating followed by nighttime cooling) cause repeated thermal expansion and contraction of the steel coil.
This uneven thermal stress cycle can induce or exacerbate sheet shape problems, such as the generation of new stress warping.
If the coil already has internal residual stress, this thermal cycle may release it, leading to further deterioration of the sheet shape.
3.What other performance changes have been made?
Surface Appearance Changes: Even without severe rust, prolonged exposure to sunlight can cause oxidation, water stains, and dirt spots on the surface, affecting the product's appearance.
Deterioration in Bonding Performance: Rusty or aged surfaces can severely impact welding quality and adhesive bonding strength.
4.What is the performance degradation process?
Prolonged exposure to sunlight → failure of the protective layer → corrosion of the substrate + thermal cycling stress → deterioration of surface quality, reduction in thickness, worsening of plate shape, and potential decrease in plasticity → ultimately leading to the downgrading or even scrapping of the material.
5.What are some warehousing recommendations?
**Absolutely Preferred:** Indoor storage, kept dry and well-ventilated.
**If Short-Term Outdoor Storage is Necessary:**
**Strict Covering:** Completely cover the material with thick, rainproof, UV-resistant tarpaulins, ensuring the tarpaulins are well-supported and drain quickly to prevent water accumulation. Ideally, a support frame should be placed under the tarpaulin to create an air circulation layer.
**Elevation and Isolation:** The roll material must be elevated using wooden blocks, kept away from damp ground, and kept level.
**Definitely 'Short-Term':** Limit storage to a few weeks if possible, and closely monitor the weather and the condition of the roll material.
**Priority Use:** Follow the "first-in, first-out" principle, and use or move the outdoor-stored roll material indoors as soon as possible.